1. Field of invention
This invention relates to the determination of formation porosity using neutron measurements.
2. Background Art
In hydrocarbon exploration and production, it is important to determine whether an earth formation contains hydrocarbon and how much hydrocarbon is in the formation. Neutron “porosity” tools are traditionally used to determine the amount of hydrocarbon and water present in pore spaces of earth formations.
A neutron tool contains a neutron-emitting source (either a chemical source or a neutron generator) and one or more axially spaced detectors that respond to the flux of impinging neutrons resulting from the interactions of neutrons with nuclei within the borehole and formation in the vicinity of the borehole. The basic concept of a neutron porosity tool is predicated on the fact that (a) hydrogen is the most effective moderator of neutrons and that (b) most hydrogen found in earth formations is contained in liquid in the pore space of the formation, either as water or as liquid hydrocarbon or gas. For neutrons emitted with a fixed energy by the source, the count rates recorded by the neutron detectors decrease as the volumetric concentration of hydrogen (e.g., porosity) increases.
FIG. 1 shows a simplified schematic illustrating a wireline neutron logging operation. As shown in FIG. 1, a neutron tool 11 is disposed in a wellbore 12. The neutron tool 11 includes a neutron source 13 and one or more neutron detectors 14. The neutron source, which may be a chemical source or an electronic neutron generator, emits neutrons into the formation 15 surrounding the wellbore 12. The emitted neutrons traverse the formation 15 and interact with matter in the formation. As a result of such interactions, the neutrons lose some of their energy. Consequently, the neutrons may arrive at the detector 14 with lower energies. By analyzing the response of the detectors to these neutrons, it is possible to deduce the properties of the surrounding formations. Although discussed by way of example in terms of a wireline tool, it should he noted that the disclosed subject matter may be employed in a while drilling environment. For example, FIG. 1b illustrates a detector neutron porosity device embodied as a logging-while-drilling tool. In this example, a source of fast neutrons 1, a near detector 2 and a far detector 3 are positioned within a drill collar. Various other configurations of detector(s) are also contemplated. The LWD tool 4 is suspended by means of a drill string 5 within a borehole 6 penetrating an earth formation 7 via action of the drill bit 8.
Since neutrons interact with hydrogenous materials, borehole fluids will interfere with neutron measurements. To correct for borehole effects, two detectors are typically used; one at a shorter spacing from the neutron source and the other at a longer spacing. With the dual detectors, it becomes possible to compensate for the borehole effects. Typically, count rate ratios between the count rates detected by the near and far detectors are used to provide a more accurate measurement of formation porosity. Examples of dual detector neutron tools are described in U.S. Pat. No. 3,483,376 and U.S. Pat. No. 5,767,510.
Traditional tools with chemical sources are able to measure the porosity of a formation in the form of a thermal neutron porosity reading. The chemical source typically relies on (α,Be) reactions in an 241 AmBe mixture. Beryllium releases a neutron of approximately 4 MeV when struck by an alpha particle, which is produced by the americium. These high-energy neutrons interact with nuclei in the formation and become slowed mainly by elastic scattering to near thermal energies. The slowing-down process is dominated by hydrogen. At thermal energies, the neutrons diffuse through the material until they undergo thermal capture. Capture is dominated by hydrogen and other thermal neutron absorbers.
Some modern neutron tools are equipped with electronic neutron sources (minitrons). In a typical electronic neutron source, deuterium (2D) and tritium (3T) ions are accelerated towards a target containing the same isotopes. When 2D and 3T collide, they react to produce high-energy neutrons (about 14 MeV). These high-energy neutrons, when emitted into formations, interact with matter in the formations and gradually lose energy. This process is referred to as slowing down. The slowing-down process is dominated by hydrogen, and is characterized by a slowing-down length (Ls). By measuring neutrons at epithermal energies, rather than thermal energies, the response provides a better estimate of hydrogen index, unaffected by thermal absorbers. Thermal neutrons typically have an average energy corresponding to a kinetic energy of 0.025 eV at room temperature, while epithermal neutrons typically have energies corresponding to kinetic energies in the range of 0.4-10 eV. However, some epithermal neutrons may have energies as high as 1 keV. One of ordinary skill in the art would appreciate that these energy ranges are general guidelines, rather than clear-cut demarcations
FIGS. 2A and 2B show two different examples of neutron tools: a traditional chemical source neutron tool 20 (e.g., CNL® tool from Schlumberger Technology Corp., Houston, Tex.) and an electronic neutron generator tool 21 (e.g., APS® tool from Schlumberger Technology Corp., Houston, Tex.), respectively. In a chemical source neutron tool 20 shown in FIG. 2A, the chemical source 25 includes a radioactive material, such as AmBe. The chemical source neutron tool 20 also includes a near detector 24 and a far detector 22 to provide a countrate ratio, which is used to calculate the porosity of a formation. The near detector 24 and far detector 22 are thermal neutron detectors. In addition, the tool 20 includes shielding materials 23 that prevent the neutrons generated by the chemical sources from directly reaching the detectors, minimizing the interference from the neutron source 25.
As shown in FIG. 2B, an electronic source neutron tool 21 uses an electronic neutron source 40 to produce high-energy (e.g., 14 MeV) neutrons. The high-energy neutrons emitted into formations are slowed to epithermal and thermal energies by interactions with matter in the formations. The epithermal neutrons are detected by detectors on the neutron tool 21, such as near detector 26, array detector 27, and far detector 29. As with the chemical source tool, the tool 21 includes shielding materials 42 that prevent the neutrons generated by the source from directly reaching the detectors. As noted above, by measuring epithermal neutrons, the detector responses are primarily dominated by the hydrogen content in the formation, without complication from neutron absorbers. Thus, the electronic neutron tool 21 conveniently provides measurements for hydrogen index. In addition, the neutron tool 21 may also include an array thermal detector 28 to detect thermal neutrons that returned from the formation. The epithermal neutron and thermal neutron measurements obtained with this tool can be used to derive various formation parameters.
In clean reservoir formations, the hydrogen index measured by epithermal neutron tools compares very well with traditional neutron porosity measured by thermal neutron tools. However, in shales, the epithermal hydrogen index often differs significantly from thermal neutron porosity. Even though the hydrogen index measurements, which are less susceptible to interference from neutron absorbers, can provide more accurate pore space estimates, they are not as commonly used as the thermal neutron porosity measurements obtained with chemical source tools. Because tools using chemical sources have been used in the industry much longer than electronic source neutron tools, users are more familiar with the thermal neutron porosity measurement. In addition, petrophysicists typically use thermal neutron porosity to indicate specific minerals as part of their formation analysis. However, chemical sources are less desirable due to their constant emission of radiation and strict government regulations. In addition, these chemical sources are becoming scarce. Therefore, there is a need for a method of converting measurements obtained with an electronic source neutron tool into measurements that could have been obtained with a traditional chemical source neutron tool.